In effecting organic chemical conversions, particularly nucleophilic substitution reactions, one frequently encounters the problem of bringing together reagents in sufficient concentration to attain conveniently rapid reaction rates. The classical solution to this problem is simply to use a polar aprotic solvent (e.g., acetonitrile, acetone, dichloromethane, dioxane) which can dissolve both reagents, e.g., a mutual solvent. However, the use of a single solvent is not always feasible, particularly on an industrial scale. Such solvents frequently are expensive, difficult to remove after the reaction; and may present environmental problems. The technique of phase transfer catalysis, see E. V. Dehmlow and S. S. Dehmlow, "Phase Transfer Catalysis", Verlag Chemie, Basel, (1983), pp. 1-3, 65-68, 371, provides a method which avoids the use of polar aprotic solvents and allows reactions to be carried out in mixtures of immiscible liquid phases. An article cited by Dehmlow, by P. Monsef-Mirzai and W. R. McWhinnie, Inorg. Chim. Acta, vol. 52, 211 (1981) is of general interest.
In the phase transfer technique the polar reagents are dissolved in a polar solvent, (most typically, water) and the nonpolar or organophilic reagents are used as neat liquids or are dissolved in a suitable organic solvent (e.g., toluene, xylene, diesel oil). A catalyst, typically an onium ion, particularly an alkyl-ammonium or phosphonium ion, is then added which transfers reagents from the immiscible polar phase to the organic phase where reaction occurs. The catalyst then is transferred back to the polar liquid phase by the polar reaction products and the cycle is continuously repeated. Micelles can also be formed which allow the polar reagent and organic reagent to come together in the aqueous phase.
One serious disadvantage of soluble phase transfer catalysts is that they are expensive and must be removed from the reaction mixture at some later stage. Distilling the solvents to recover the catalysts is time and energy consuming. Also, soluble phase transfer catalysts sometimes cause undesirable foaming of the reaction mixture and do not lend themselves to convenient chemical processing methods.
Heterogeneous phase transfer catalysis, sometimes called triphase catalysis, has been developed to solve these problems, see S. L. Regen, J. Amer. Chem. Soc., vol. 98, 6270 (1976). From the viewpoint of industrial applications, heterogeneous phase transfer catalysts are very attractive because they are easily recovered by filtration and because they are ideally suited for continuous flow processing methods. The heterogeneous phase transfer catalysts developed to date make use of polymers (e.g., polystyrene) see S. L. Regen, J. Amer. Chem. Soc., vol 97, 5956 (1975) and J. Org. Chem., vol. 42, 875 (1977), also M. Schneider et al, J. Org. Chem., vol. 47, 364 (1982), or metal oxides (e.g., silica, alumina) see P. Tundo et al, J. Amer. Chem. Soc., vol. 104, pp. 6547 and 6551 (1982). These solids have been functionalized so that polar reagents, particularly anions, can bind electrostatically to their surfaces. Also, their surfaces are sufficiently organophilic so that organic reagents will adsorb on their surfaces and undergo reaction with the immobilized polar reagent.
Polymer-based heterogeneous phase transfer reagents are sometimes limited by diffusion of reagents into and out of the polymer matrix. Also, swelling of the polymer matrix by the reagents or products can make it difficult to regulate the diffusion process. Even under the best of technical conditions, polymer-based catalysts suffer the economic disadvantage of high manufacturing costs. Oxide-based phase transfer catalysts also are expensive to manufacture. To functionalize oxide surfaces with appropriate alkylammonium or alkylphosphonium ions, one must covalently link the ions to the surface through the use of a coupling agent, usually a silane. These functionalized oxides often are not easily dispersed in the reaction mixture and reaction rates are often limited by relatively low surface areas.
In formulating a heterogeneous phase transfer reagent, one needs to optimize the surface area of the interfacial region between the solid catalyst and the immiscible liquid phases, because it is this interface which is important in bringing together the reacting reagents. FIG. 1 illustrates this concept. Y.sup.- represents the water soluble polar reagent or reactive species and RX is the organophilic reagent contained in the immiscible organic phase. The function of the heterogeneous or solid phase is to bring the reagents together to form RY and X.sup.- as products in the interfacial region between the solid catalyst and the liquid phases.
Kadkhodayan and Pinnavaia in Journal of Molecular Catalysis, vol. 21, pp. 109-117 (1983) have previously demonstrated that intercalated smectite clays were useful heterogeneous phase transfer reagents. In these systems the interlayer regions were occupied by layers of organometallic cations and inorganic anions. One severe limitation, however, was that the intercalated salt was eventually desorbed from the clay interlayers and lost to solution, the advantages of a solid state phase transfer catalyst thereby being lost. Cornelis and Laszlo in Synthesis, pp. 162-163 (February 1982) used a commercially available organocation clay (Tixogel) as a phase transfer catalyst for a specific type of organic conversion.
In general, the present invention concerns a method in which organoclays, e.g., cation exchanged forms of swelling 2:1 layered silicate clays containing alkylammonium, alkylphosphonium or related "onium" ions, provide efficient interfacial surfaces for the catalysis of organic conversions in which the reagents and products are partitioned between two or more immiscible liquid phases.